During the installation or subsequent upgrade of an HVAC system, there are occasions when wires may be incorrectly connected to various components of the HVAC system, including an electronic thermostat. When wires are incorrectly connected to a thermostat, there is the possibility for a short circuit to be created that if not accounted for could result in permanent damage to either the thermostat, the HVAC wiring and/or other HVAC system components. In order to protect against such conditions, the electronic thermostat can include one or more fuses that are designed to blow under the increased current of a short circuit condition. However, blown fuses are problematic from support and customer satisfaction viewpoint. The problem of blown fuses can be particularly problematic for thermostats during installation, and problems can be difficult to diagnose and remedy.
Additionally, switching circuitry in a thermostat can also be used to actuate, or activate, HVAC functions, such as air conditioning, heating, and/or fan operations. When a common (“C”) wire is not available, modern advanced thermostats may steal power from HVAC call signal wires at levels low enough to not interfere with normal HVAC functions, but high enough to charge an energy storage element, such as a rechargeable battery or large capacitor. Thus, modern thermostat designers should design switching circuitry that can both protect the thermostat from voltage/current anomalies while still enabling power stealing functionality.